1. Field
The present disclosure relates generally to electronics, and more specifically to transmitters and receivers.
2. Background
In a radio frequency (RF) transceiver, a communication signal is typically received and downconverted by receive circuitry, sometimes referred to as a receive chain. A receive chain typically includes a receive filter, a low noise amplifier (LNA), a mixer, a local oscillator (LO), a voltage controlled oscillator (VCO), a baseband filter, and other components, to recover the information contained in the communication signal. The transceiver also includes circuitry that enables the transmission of a communication signal to a receiver in another transceiver. The transceiver may be able to operate over multiple frequency ranges, typically referred to a frequency bands. Moreover, a single transceiver may be configured to operate using multiple carrier signals that may occur in the same frequency band, but that may not overlap in actual frequency, an arrangement referred to as non-contiguous carriers.
In some instances, it is desirable to have a single transmitter or receiver that is configured to operate using multiple transmit frequencies and/or multiple receive frequencies. For a receiver to be able to simultaneously receive two or more receive signals, the concurrent operation of two or more receive paths is required. Such systems are sometimes referred to as “carrier-aggregation” systems. The term “carrier-aggregation” may refer to systems that include inter-band carrier aggregation and intra-band carrier aggregation. Intra-band carrier aggregation refers to the processing of two separate and non-contiguous carrier signals that occur in the same communication band. Currently, even though these non-contiguous carriers may be close together, a separate receive chain is typically needed to process each carrier. When implementing such a carrier aggregation receiver, it is possible that power from one carrier may interfere with the downconversion of another carrier (causing what is referred to as intermodulation distortion, and when occurring as a second-order function, is referred to as IM2) that can lead to desensitizing the receiver, a condition sometimes referred to as “receiver desensitization” or “receiver desense.” Receiver desensitization may occur due to the presence of second-order distortion caused by the operation of the low noise amplifier (LNA). The second order intercept point (IIP2) refers to a measure of linearity that quantifies the second-order distortion generated by non-linear systems and devices, in this example, the LNA.
At low power levels, the fundamental output power of the LNA rises in a one-to-one ratio (in terms of dB) with respect to the input power, while the second-order output power rises in a two-to-one ratio. If the input power is high enough for the LNA to reach saturation, the output power flattens out in both the first- and second-order cases.
The second order intercept point (IIP2) is the output power point at which the extrapolated first- and second-order lines intersect on a plot, since the actual power levels will flatten off due to saturation at much lower power level typically.
It would be desirable to reduce receiver desensitization caused by second-order non-linearities, particularly those caused by the LNA.